As a writing instrument of the type of directly storing liquid ink as described above, as disclosed in JP 2001-315483, there is known a writing instrument in which a circular through hole is formed in the central portion of a partition wall that divides an ink storing chamber and a reservoir chamber, while into the through hole is inserted an ink supply member (relay core that connects a writing element and the ink storing chamber) formed of a rod-like member circular in cross section comprised of porous material. In this structure, a diameter of the through hole is formed to be larger than a diameter of the ink supply member, and a gap capable of holding the ink by capillary force is formed between the ink supply member and an inner wall of the through hole.
The through hole is in a state where an ink membrane (seal) is formed between the outer circumference of the ink supply member impregnated with the ink and the hole by capillary force. The ink membrane of the through hole breaks due to increases or decreases in pressure inside the ink storing chamber, and so-called gas-liquid exchange process is obtained such that the ink flows into the reservoir chamber or air flows into the ink storing chamber. Such gas-liquid exchange process is preferable in structure in terms of compensation for variations in ambient temperature and in pressure inside the ink storing chamber, and the ink membrane is formed in the central axis portion of the ink storing chamber, and therefore, the least prone to influences due to variations in attitude of the writing instrument, whereby seal characteristics are stabilized and it is restricted that the ink flows out of the ink storing chamber to the reservoir chamber accidentally. Further, when the ink is consumed by writing, ink is sucked out of the portion where the ink is held by caterpillar force, and the gas-liquid exchange process is carried out at this portion. Therefore, the ink is supplied to the writing element with high sensitivity, thus providing the writing instrument of structure where light and dark hardly occurs in writing.
The ink supply member is positioned so that a predetermined gap is formed between the outer circumference of the ink supply member and the inner wall of the through hole formed in the partition wall by press-fitting a back end portion of the supply member to a holding member provided on a back end of the main body of the writing instrument to engage therein, and attaching a writing-element portion provided on a front end portion of the ink supply member to the main body of the writing instrument.
In the writing instrument with the aforementioned structure, it is important to control dimensions of the diameter of the ink supply member and of the diameter of the through hole formed in the partition wall. More specifically, as shown in FIG. 1A, a difference between an outer diameter D1 of an ink supply member 1 and a diameter D2 of a through hole 2a formed in an partition wall 2 is set at about 0.1 mm (i.e. when D1 is 2 mm, D2 is set at 2.1 mm). In other words, in terms of dimensions of the gap, the gap of substantially 0.05 mm is formed around the outer circumference of the ink supply member.
When the gap is excessively wide (more than or equal to 0.2 mm), the ink flows out irrespective of types of ink. Meanwhile, when the gap is excessively narrow, supply of the ink cannot catch up with writing speed, and light and dark (ink squeeze out) occurs on characters. Further, the expansion coefficient of the ink supply member varies with the material of the ink supply member and the ink (such as oil-based ink and water-based ink) to use. Therefore, when the gap is formed narrowly in advance, the ink becomes stuck and/or light and dark occurs on characters. Accordingly, in preparing the writing instrument with the above-mentioned structure, it is important to control dimensions of the ink supply member and of the through hole formed in the partition wall.
When the ink supply member is actually incorporated into the main body of the writing instrument, depending on conditions of press fitting of the end portion, it sometimes happens that the axis of the ink supply member deviates as shown in FIG. 1B, and the gap changes. In this case, since there are preparation errors to some extent in the diameter of the ink supply member and in the diameter of the through hole formed in the partition wall, the gap G1 generated by the axis deviation may be 0.2 mm or more. When the gap thus becomes 0.2 mm or more, the ink flows out, and there arise possibilities such that writing becomes unable before the ink is completely used and that the ink leaks. Particularly, in the structure where the ink storing chamber is partitioned using a plurality of partition walls, when distortion occurs on the ink supply member, the aforementioned problem tends to arise in either partition wall portion. Accordingly, the precision in incorporating the ink supply member is important in preparing the writing instrument with the above-mentioned structure.
As described above, the structure of well-known technique requires precise dimension control of constituent members and incorporation technique of high precision, and has the problem that fluctuations in quality are apt to occur for each product due to dimension error and/or distortion of the ink supply member caused by incorporation.
In view of the foregoing, it is an object of the present invention to provide a writing instrument which is easy in manufacturing and has a structure such that fluctuations hardly occur in quality, where the writing instrument has the structure in which an ink storing chamber and a reservoir chamber are divided by a partition wall, while an ink supply member that supplies ink is inserted into the partition wall with a predetermined gap kept.